Rod screen



y 2, 1968 PER A. H. HzSON FAHLSTROM 3,390,770

ROD SCREEN.

Filed June 18, 1965 INVENTOR- er H.151]; H-son Fa H slzrom United States Patent 3,390,770 ROD SCREEN Per A. H. H:Son Fahlstrom, Boliden, Sweden, assignor to Boliden Aktiebolag, Stockholm, Sweden Filed June 18, 1965, Ser. No. 465,044 Claims priority, application Sweden, June 24, 1964, 7,721/64 8 Claims. (Cl. 209-433) ABSTRACT OF THE DISCLOSURE A process and apparatus for separating a mass of particles into fractions whereby the mass to be treated is fed to a bed of rods which are freely movable relative to one another. The rods form a three dimensional screen and, when vibrated, separate the particles.

The present invention relates to a method of separating fine particles into fractions according to absolute grainsize by screening said particles between rods moving in relation to one another. The invention is more especially related to finely pulverized dry or slurried ore material subjected to upgrading.

It is known that is technically feasible to screen down to 0.2 mm., but with low capacity and excessive wear of the screen. There is a need within the art of concentration, however, to screen particulate material on a screen having a mesh of around 0.2 mm. or less.

During flotation processes it is often desired to separate particles within the range of 005-02 mm. from particles which are both finer and coarser. Usually this is accomplished with the assistance of classifying equipment, i.e. equipment in which separation is effected on the basis of the falling velocity of the particles in a liquid. Such equipment does not permit clear separation and is consequently unsatisfactory in many cases. Screening is par ticularly desirable for size classification in conjunction with grinding. If classification in a grinding circuit is carried out with the aid of classifying equipmenti.e. separation into fractions whose constituent particles have similar settling velocities-fractions are obtained which are not free from oversize or undersize, that is to say fine fractions contain coarse light particles exceeding the nominal separation size whereas coarse fractions contain fine heavy particles finer than the nominal separation size. Undersize is already ground to a satisfactory degree and ought not to be included in the coarse product. Oversize in the fine fraction can often contain valuable minerals in the form of middlings grains and thus is not ground sufliciently. It is desirable to be able to return the coarse middlings grains from the fine fraction to a grinding circuit. The use of classifiers or wet sorters alone does not permit this without simultaneously subjecting the whole product to a further grinding process, and consequently reducing the average particle size of the whole product. By means of screening-a separation according to abso lute grain size-it is, however, possible to separate the coarse particles and to return them to, for example, a grinding circuit.

According to the present invention the particles are divided up according to particle size in a vibratory screen provided with a bed of freely lying rods between which the charge passes. According to one embodiment of the invention the rods lie longitudinally in a sloping-preferably rectangular-screening frame, the bottom of which is provided with a number of support members. Collection pockets for various fractions are located under the frame. The mean grain size of material passing through the rod bed increases with the distance from the feed end of the screen. The bed is made up of a number of layers "ice of rods e.g. say 10-26 layers. The height of the bed is increased as the grain size distribution in the feed material becomes wider. The length of the rods is selected according to the screening capacity and the degree of fractionation required. The diameter of the rods is usually from 5 mm. to 12 mm. with fine screening (less than 0.2 mm.). Rods used in coarse screening are proportionally larger in diameter. The screen now being described is self-cleaning and when rubber covered rods are used, material wear is very slight clue to the elastic covering and the resiliency of the rod bed. Various embodiments of the screen are possible, for example the described screen frame where the charge is fed from the top with or without over-pressure. The charge can also be fed from underneath under pressure. In this latter example the coarsest fraction falls at that point farthest away from the inlet end, each of the particle fractions being collected and led away separately from the upper surface of the rod screen.

According to the invention it is also possible to arrange the rods at right angles to the longitudinal direction of the screen and to use longitudinal support beams in the sloping screen frame for the purpose of supporting said rods.

Another possible arrangement of the screen rods according to the invention is to position the rods alternately in the longitudinal and lateral direction of said screen frame thus forming a three-dimensional gridwork of movable rods.

The separating ability of the screen can be adapted within wide limits by suitable choice of amplitude, frequency and direction of the vibrating movement which is imparted to the rod screen. When an eXtrernely fine screening is desired amplitudes in the range of 0.5-5 mm. and frequencies of 1000-3000 vibrations per minute are usually selected. The desired result can also be achieved in a slowly rotating and possibly vibrating barrel provided with a bed of rods onto which the material is fed from one end and from above without being subjected to pressure.

Finely divided material can easily be screened, using the arrangement according to the invention, down to 0.05 mm. With an initial change having a maximum particle size of for example 0.5 mm., particles over 0.1 mm. can be separated for regrinding whereas the finer particles are separated into fractions from 0.05 to 0.1 mm., which is the common requirement within the flotation technique. The described invention is therefore, particularly useful in grinding circuits concerned with the fine grinding of ore.

The invention will be more closely described in the following with reference to the attached drawing on which FIGURE 1 diagrammatically shows a longitudinal section through one embodiment of a rod screen according to the invention, FIGURE 2 shows a cross section through the rod screen according to FIGURE 1 and FIGURE 3 shows one example of how two rod screens according to the invention can be included in a grinding circuit. The rod screen shown in FIGURES 1 and 2 consists of a sloping screen box having two parallel side walls 1, 2, two end walls 3, 4, one roof 5 and a screen bottom 6 of a type known per se, e.g. a fine mesh wire net stretched onto a steel frame. A number of support members 7, arranged on the screen bottom 6 laterally to the rod screen, serve as supports for a filling comprised of screen rods 8, which in the shown embodiment extend in the longitudinal direction of the screen box. A vibrator 12 of a type known per se is arranged on a support structure projecting from the side walls 1, 2 of the screen box, which support structure consists of side walls 9 and 10 and a transverse beam 11, and which vibrator 12 is constructed for a frequency of for example 1000-3000 vibrations per minute and an amplitude of 0.5-5 mm. A number of collccting pockets 1317 intended for gathering fractions of the screened material is situated under the screen bottom 6 immediately adjacent to the same but without coming into direct contact with it.

On operating the screen the vibrator 12 is set into motion after which the material to be screened is fed into the screen box in the direction of arrow 18. The fine charge endeavours to take the shortest route down through the screen bed and is collected in the pocket 13 adjacent to the feeding position whereas particle fractions of increasing grain size are collected in turn inthe following pockets whereby the coarsest fraction is collected in the lowest pocket 17to the right in FIGURE 1. Five collecting pockets have been selected in the shown embodiment but naturally the number of pockets can be varied. As a rule at least two pockets are preferred in spite of the fact that in principle it is also possible to arrange only one pocket and to allow the oversize to pass out from the screen box through an outlet disposed in the end wall 4 of said box.

FIGURE 3 shows a schematic drawing of a screening system including two rod screens according to the invention included in a closed wet grinding circuit. An aqueous pulp consisting of ore particles intended for grinding is fed through an inlet conduit 19 into a mill 20. The ground pulp is then passed from the outlet 21with the assistance of a pump 22-through a conduit 23 to a first rod screen 24 and is there separated into a fine fraction-collected in a pocket 25-and a coarse fractioncollected in a pocket 26. The coarse fraction is returned from pocket 26 via a return conduit 27 to the mill inlet 19 and is combined with the pulp fed into the mill. The fine fraction is passed from pocket 25 through a conduit 28 to a second rod screen 29 of similar construction to rod screen 24, where it is separated into a finished product which is collected in a pocket 30 and led away through an outlet conduit 31 for further treatment, whereas the coarse fraction is collected in a pocket 32 and returned to the mill inlet 19 via a return conduit 33.

By means of the described screening system a finished pulp is obtained the particles of which are screened to sizes within a very closely limited particle size range, ie with a distribution not possible with conventonal cyclone separaton.

The drawings only show representative embodiments of the rod screen according to the invention and how it can be used in a two-stage process in a grinding circuit. As was mentioned earlier it is possible to provide the rod screen with several collecting pockets from which thevarious fractions can be directed to different desired points in the grinding circuit. The rod screen can also be combined with other separating devices of types known per se, for example cyclones and common vibratory screens.

The rod screen according to the invention can be used for dry screening as well as wet screening although it has its greatest usefulness when wet screening ore material. When concerning dry screening it is also within the scope of the invention to feed in the granular charge to be screened with compressed air from the underside of the screen and to arrange in the roof of the screen box a number of suction hoods distributed along the longitudinal direction of said screen box for collecting the airborn particle fractions.

It is also possible according to the invention-instead of the rods 8 placed in the longitudinal direction of the screen box-to arrange rods lying at right angles to the longitudinal direction of said box and to use longitudinal support members for supporting the same.

As was previously mentioned it is also possible to place the rods lengthwise and crosswise thereby obtaining a three-dimenional gridwork of movable rods.

What is claimed is:

1. An apparatus for separating a fine particulate mate rial according to particle size, comprising a box extending substantially horizontal and having two side walls and two end Walls; a screen support member disposed in said box at substantially the bottom thereof; a plurality of rods freely arranged on said support member in successive contacting layers, each of said layers comprising rods which are contiguous and in parallel relationship, and which occupy the full extent of the distance between said end walls and said side walls; and vibrating means connected to said box to cause the rods to move relative to each other and to said support member to permit a portion of material introduced into said box from above said rods to pass through said'rods and said support member.

2. The apparatus according to claim 1, wherein said support member slopes in the general longitudinal direction of said box.

3. The apparatus according to claim 1, wherein said rods rest on said support member with their axes extending substantially parallel to the longitudinal axis of said support member.

4. The apparatus according to claim 1, wherein said rods rest on said support member with their axes arranged substantially horizontal and extending at substantially right angles to the longitudinal axis of said support memher.

5. The apparatus according to claim 1, wherein the rods of successive contacting adjacent layers extend at substantially right angles to each other.

6. The apparatus according to claim 1, wherein said support member slopes in the longitudinal direction of said box and is divided transversely into at least two sections, each having rod layers disposed the-rein, said rods successively decreasing in diameter from section to section in the downward longitudinal direction of said box, said material being introduced over the uppermost end wall of said support member.

7. Theapparatus according to claim 1, wherein said rods are sheathed in rubber.

8. The apparatus according to claim 1, further comprising at least one receiving hopper under said support member for collecting the material passing therethrough.

References Cited UNITED STATES PATENTS 770,079 9/ 1904 Lamb 209-393 1,031,659 7/1912 Muender 209466 1,905,417 4/1933 MacDonald 241-46 2,068,783 1/1937 Wendell 20944 2,080,884 5/1937 Anderson 209-896 X 2,922,588 1/1960 Hoesch 209-323 X 3,064,812 11/1962 Wehner 209379 X OTHER REFERENCES Eng. & Min. Journal, 161, No. 7, 88-91, July 1960.

HARRY B. THORNTON, Primary Examiner. R. HALPER, Assistant Examiner. 

